Substantial recent progress has been made by the assignee of the present invention in the development and refinement of miniaturized, compact, hand-held precision x-ray sources including integral precision power supplies (known as "generators" in the x-ray art). The reader is referred to the commonly assigned prior U.S. Pat. Nos. 4,646,338 and 4,694,480, and the disclosures thereof are expressly incorporated herein by reference thereto as examples of the assignee's ongoing developments in this art.
While the hand held precision x-ray source described, for example, in the referenced U.S. Pat. No. 4,694,480 worked very well, particularly for spectrometric data collection uses and environments requiring continuous generation of X-rays of precise energy and intensity over an extended time period, that prior source has manifested several drawbacks in some desirable applications and operating environments. First, that prior unit was limited to generating x-ray energy from high voltage levels of no greater than fifty kilovolts, and x-ray intensity levels from beam current levels of only one hundred milliwatts. For many useful applications, including imaging applications, the relatively low x-ray power level provided by the referenced prior unit failed to result in sufficiently energetic x-ray generation and failed to provide satisfactory images, particularly in dental and chest x-ray applications.
Also, in radiographic imaging applications; the requirement of constant x-ray excitation was not present, and a pulsed excitation source became essential from a safety and operating point of view. In addition, when used with electronic imaging systems for digital image storage or for direct viewing on a television screen, it has become sometimes desirable to have the x-ray pulse period as short as one millisecond with the pulse commencement capable of being synchronized with the imaging system with which the pulsed source is being used.
A related drawback of prior approaches to pulsed x-ray generation having controlled energy levels relates to x-ray tube construction. With grid controlled x-ray tubes, unless care is given to the design, manufacture and operating conditions of the x-ray tube, application of high beam acceleration voltage across the electron gun assembly and the target anode tends to cause stray electron emission from cathode material which has migrated to the heated control grid and focus grid elements of the electron gun assembly, even though a negative beam-cutoff potential has been applied to the control grid. Stray electron emission results in unwanted and uncontrolled x-ray generation, leading directly to imprecision and safety issues in small, handheld portable x-ray sources.
Another drawback of the cited prior hand-held unit was its relative weight which was directly attributable to extensive external lead-sheet x-ray shielding required at and around the x-ray tube in order to eliminate any stray x-ray photon radiation.
Another drawback of the referenced prior art approach was the lack of a circuit within the precision x-ray generator to provide a precise, controllable time increment for the x-ray pulse duration.
A further drawback of the common-assignee's referenced prior art unit was that it was not sufficiently ruggedized to withstand potentially rough handling necessarily incident to field use and handling of any hand-held apparatus.
Other workers have proposed pulsed portable x-ray sources as taught, for example, by the Golden U.S. Pat. No. 3,878,394, and the Dyke et al. U.S. Pat. No 3,256,439. However, those prior approaches used spark gap and transmission line generator technology and thus were not capable of providing x-rays of precisely controllable duration, intensity or energy.
Ruitberg et al. U.S. Pat. No. 4,517,472 described an x-ray generator for exciting an x-ray tube fitted with a focus grid in order to control electron beam width and shape. Although three separate DC switching power supplies were used, the Ruitberg et al. approach did not provide any beam gating grid control and therefore was not a pulsed x-ray beam system.
Thus, a hitherto unsolved need has arisen for a truly portable, battery operated x-ray generator capable of generating x-rays of presettable energy, intensity and precisely controllable duration.